1. Field of the Invention
This invention relates to automatic gain control circuits and more specifically to improvements in such circuits particularly suited to the reception of pulse type signals, for instance, in digital magnetic recording readback systems.
2. Description of the Prior Art
An automatic gain control circuit typically comprises a gain-controlled amplifier for receiving a variable amplitude input signal to produce a substantially constant amplitude output signal which is applied to a feedback circuit. The feedback circuit includes a peak detector that produces a feedback voltage for comparison with a reference voltage to generate a control voltage for setting the gain of the gain-controlled amplifier so that the magnitude of the output signal is held substantially equal to a desired amplitude. The peak detector includes a capacitor circuit which operates with a charging rate which permits the peak detector to follow closely the peak amplitudes of the output signal. The discharging rate of the capacitor circuit is made slower than the changing rate so that the capacitor does not fully discharge between successive peaks of the input signal, thus enhancing the ability of the peak detector to follow the peaks of the output signal. A readback circuit is usually coupled to the output of the automatic gain control circuit and operates to detect data which is represented by the locations of the peaks in the output signal.
While such a circuit has been used with success in analog-signal detection systems, certain limitations occur in the ease of pulse type signal detection systems wherein the input signal may be expected to be absent during certain periods. In a disk drive system, for example, wherein data is recorded on a plurality of concentric tracks on a recording disk, the data is arranged along each track such that segments, known as address marks, occur which contain no data. Furthermore, portions of the disk surface may have imperfections which will result in the data recorded on those portions having such low amplitudes that the data cannot be detected by the readback circuit. Such loss of data is known in the disk drive art as data drop-out. In any case such as address mark or data drop-out where the input signal is absent for some period, the gain of the gain-controlled amplifier will be substantially higher than a nominal gain level because the automatic gain control circuit attempts to increase the amplitude of the output signal from essentially zero to the desired amplitude by raising the gain quickly to a large value. Upon the reappearance of the input signal at a nominal peak amplitude, the automatic gain control circuit will produce an output signal which is substantially greater in amplitude than that which would be produced if the gain were at the nominal gain level. The capacitor circuit of the peak detector charges rapidly in response to the abrupt increase in the output signal voltage. A large amplitude feedback voltage is thus produced which results in the gain of the gain-controlled amplifier to be reduced at a rapid rate in an effort by the automatic gain control circuit to restore the output signal voltage to the desired amplitude. However, the capacitor circuit of the peak detector cannot respond as fast as the reduction in the output signal voltage because as explained above, the discharge rate is slow. As a result, the amplitude of the feedback voltage produced by the peak detector will not closely follow the amplitude of the output signal. Therefore, the gain reduction of the gain-controlled amplifier will not be suitably controlled and the amplitude of the output signal is likely to fall below (that is, undershoot) the desired amplitude before the peak detector is able to produce the proper feedback voltage to stop the gain reduction. Undershooting is undesirable because some data will be lost if the output signal peaks are reduced to a level too low to be detected by the readback circuitry coupled to receive the output signal. Such data loss is known as output signal dropout.